Ant–plant mutualism: a dietary by‐product of a tropical ant's macronutrient requirements

Abstract

Many arboreal ants depend on myrmecophytic plants for both food and shelter; in return, these ants defend their host plants against herbivores, which are often insects. Ant-plant and other mutualisms do not necessarily involve the exchange of costly rewards or services; they may instead result from by-product benefits, or positive outcomes that do not entail a cost for one or both partners. Here, we examined whether the plant-ant Allomerus octoarticulatus pays a short-term cost to defend their host plants against herbivores, or whether plant defense is a by-product benefit of ant foraging for insect prey. Because the food offered by ant-plants is usually nitrogen-poor, arboreal ants may balance their diets by consuming insect prey or associating with microbial symbionts to acquire nitrogen, potentially shifting the costs and benefits of plant defense for the ant partner. To determine the effect of ant diet on an ant-plant mutualism, we compared the behavior, morphology, fitness, stable isotope signatures, and gaster microbiomes of A. octoarticulatus ants nesting in Cordia nodosa trees maintained for nearly a year with or without insect herbivores. At the end of the experiment, ants from herbivore exclosures preferred protein-rich baits more than ants in the control (i.e., herbivores present) treatment. Furthermore, workers in the control treatment were heavier than in the herbivore-exclusion treatment, and worker mass predicted reproductive output, suggesting that foraging for insect prey directly increased ant colony fitness. The gaster microbiome of ants was not significantly affected by the herbivore exclusion treatment. We conclude that the defensive behavior of some phytoecious ants is a by-product of their need for external protein sources; thus, the consumption of insect herbivores by ants benefits both the ant colony and the host plant.

Insights Into the Ecological Role of Pseudomonas spp. in an Ant-plant Symbiosis

“…Myrmecophytic interactions are mutualistic relationships between ants and plants, in which nutrient exchanges and protection of both species are involved (Fonseca, 1999;Webber et al, 2007;Dejean et al, 2012). Some of these interactions are well studied, such as Acacia-Pseudomyrmex, Tachigali-Pseudomyrmex, Leonardoxa-Petalomyrmex, Piper-Pheidole, Macaranga-Crematogaster, Cordia-Allomerus, and Cecropia-Azteca (Heil and McKey, 2003;Voglmayr et al, 2011;González-Teuber et al, 2014;Mayer et al, 2014;Hernández et al, 2017). Myrmecophytes offer shelters (hollow stems that provide nesting space) and food to associated ants through the production of structures such as glycogenrich Müllerian corpuscles, pearl bodies, and extrafloral nectaries (Davidson and Fisher, 1991;Oliveira et al, 2015).…”

Induction, engineering, and hijacking of defensive strategies of the host by a gall‐inducing weevil

“…In most cases, these structures are ontogenetically derived from leaf tissues, although they can also be derived from shoot epidermal cells, trichomes, and other structures at the plant surface (Gonz alez-Teuber and Heil 2015). Plants not aided by mutualistic ants may suffer almost twice as much damage from herbivores, which often results in decreased fitness (Rosumek et al 2009, Arcila-Hern andez et al 2017.…”